1. Field of the Invention
The present invention relates to an Fe-Cr-Mn alloy for constituting a reactor for a nuclear fusion reactor, a fast breeder reactor, a light-water nuclear reactor or the like. More particularly, the present invention relates to an Fe-Cr-Mn alloy capable of satisfactorily preventing the lowering of concentration of chromium at grain boundaries when subjected to a neutron irradiation environment.
2. Prior Art
Hitherto, the Fe-Cr-Mn alloy, which has been developed as the material for use as the atomic reactor material, is constituted by substituting nickel with manganese, nickel being a main component of the Fe-Cr-Ni alloy, which had been widely employed as the steel for the light-water nuclear reactor or the fast breeder reactor. Furthermore, element compositional proportion of the alloy has been changed or minor elements have been added to the alloy for the purpose of securing the phase stability. Therefore, an advantage can be obtained in that residual radioactivity (to be called "induced radioactivity" hereinafter) of the radioactive isotope, formed by irradiation of neutrons having the energy spectrum generated due to fusion reaction, can be reduced. Therefore, safety of the nuclear fusion reactor can significantly be improved and an excellent economical advantage can be obtained in terms of facility of the maintenance of the reactor and efficiency of the waste disposal and re-utilization of reactor systems.
In Japanese Patent Unexamined Publication No. 61-9560, a conventional alloy is disclosed, which has fine austenitic structure and which is composed of 20 to 40 wt% of manganese, 0 to 15 wt% of chromium, 0.4 to 3.0 wt% of silicon, at least one of less than 0.7 wt% carbon and 0.3 wt% of nitrogen each of which quantity can stabilize fine austenitic structure, 0 to 0.1 wt% of phosphor, 0 to 0.01 wt% of boron, 0 to 3.0 wt% of aluminum, 0 to 0.5 wt% of nickel, 0 to 2.0 wt% of tungsten, 0 to 1.0 wt% of tantalum, 0 to 2.5 wt% of vanadium and the balance substantially composed of iron. High manganese austenitic steel having a same structure and improved high temperature strength is disclosed in Japanese Patent Unexamined Publication No. 62-238353, the high manganese austenitic steel being composed of, by weight, 0.05 to 0.5% of carbon, 12 to 50% of manganese, 2 to 20% of chromium, 0.1 to 5.0% of silicon, 0.01 to 4.0% of aluminum, 0.25% or less of nitrogen, one or more elements selected from a group consisting of 0.01 to 1.0% of titanium, 0.01 to 1.0% of niobium and 0.005 to 0.2% of zirconium by a predetermined proportion and the balance of iron except for unavoidable impurities. There is also a high manganese austenitic steel which is composed by adding at least one or more elements selected from a group consisting of 10% or less of nickel, 5% or less of cobalt and 10% or less of copper to the above-described high manganese austenitic steel. In addition, a high manganese austenitic steel of a different type is known which is composed by adding one or more elements selected from a group consisting of 5% or less of molybdenum and 5% or less of tungsten to the above-described high manganese austenitic steel. Furthermore, a high manganese austenitic steel is known which is composed by adding one or more elements selected from a group consisting of 10% or less of nickel, 5% or less of cobalt and 10% or less of copper and one or more elements selected from a group consisting of 5% or less of molybdenum and 5% or less of tungsten to the above-described high manganese austenitic steel.
However, the change in composition of the elements of the above-described alloys at grain boundaries which can take place when used under neutron irradiation environment has not been studied. In particular, the alloys have a problem of lowering of chromium concentration at grain boundaries which may cause deterioration in corrosion resistance and in strength.